Patent GB-923613-A (GB)
Title: Navigation system
IPC: G05D1/02
Filed: 1959-08-11
Abstract: 923,613. Radio navigation. BENDIX CORPORATION. Aug. 11, 1959 [Sept. 5, 1958], No. 27462/59. Class 40 (7). Relates to apparatus for asymptotically guiding a craft towards and along a radio beam in which signals proportional respectively to the displacement D of the craft from the beam axis and the craft heading # relative to the direction of the beam axis are combined to give a first control signal which is applied to an automatic pilot 30 or a manual flight director display 32. According to the present invention the ratio D/# is automatically reduced when the displacement signal D, or a further control signal which includes the displacement signal D as the predominant component thereof, falls to a first predetermined amplitude at a substantially predetermined displacement from the beam axis so that, Fig. 6 (not shown), initially the craft makes a sharp approach to the beam axis (i.e. the " intercept " mode) and then after the ratio D/# has been reduced the craft approaches the beam axis more gradually (i.e. the " tracking " mode). Fluctuations in the displacement component D of the first control signal are eliminated by a smoothing circuit and the time constant of the smoothing circuit is automatically increased if, after the ratio D/# has been reduced, the amplitude of the displacement signal D, or of said further control signal, rises above a second predetermined value greater than said first predetermined value, e.g. as the craft enters the cone of confusion over the beacon producing the radio beam. As shown, the displacement signal D produced by a radio receiver 2 is combined in a sensor unit 10 with minor corrective signals from a source 11 proportional to the craft speed, distance of the craft from the beacon transmitter and rate of change of the displacement to give the further control signal which operates a relay 8 when its amplitude falls below the first predetermined magnitude. The displacement signal D is also converted into an A.C. signal in a circuit 4 and applied through a limiter 6 and smoothing circuit 12 to a ratio-changing circuit 18 where it is combined in a transformer, Fig. 4 (not shown), with the heading signal # from a heading sensor 20, the resultant first control signal being smoothed in a softening circuit 22 and then combined with rate of turn and altitude signals from a source 24 and applied as the final control signal to the automatic pilot 30 and/or flight director 32. When the relay 8 operates to initiate the tracking mode it actuates contacts in the limiter 6, ratio changing circuit 18 and softening circuit 22 respectively to (1) decrease the limiting level of the limiter 6; (2) decrease the ratio D/# by switching potentiometers in the signal input circuits of the ratio changer 18; and (3) connect a negatively charged capacitor, Fig. 5 (not shown), to the grid of a valve in the softening circuit 22 so that the gain decreases abruptly and then increases exponentially as the capacitor discharges thereby attenuating any switching transients produced in the circuits 6 and 18. When the relay 8 has operated it remains in this condition even if the output from the sensor unit 10 subsequently exceeds the first predetermined magnitude. Operation of the relay 8 also closes contacts 16 and 28. The closure of contact 28 causes signals from a unit 26 to be combined with the final control signal to correct for wind forces on the craft; such compensation may be effected by cancelling the heading signal, integrating the displacement signal or by Doppler radar signals. The closure of contact 16 applies the further control signal from the sensor unit 10 to a relay 14 which operates when said further control signal exceeds the second predetermined magnitude which is greater than the first predetermined magnitude at which the relay 8 operates. Such a condition may arise when the craft passes through the cone of confusion over the beacon, Fig. 7 (not shown), and when this occurs the relay 14 opens a contact, Fig. 3 (not shown), in the smoothing circuit 12 to increase the time constant thereof thereby attenuating the signal fluctuations due to the cone of silence. When a known To-From indicator is included which disconnects the beam signals in the cone of confusion, the operation of said indicator is arranged to be effected at a higher signal level than that which operates the relay 14. If the ratio changer 18 is included between the circuits 4 and 6, the softening circuit may be omitted.